This invention relates to a process for converting oxide superconducting precursors into textured and densified oxide superconductor articles. This invention further relates to a method for preparing an oxide superconducting composite in a minimum of processing steps.
Superconductors are materials having essentially zero resistance to the flow of electrical current at temperatures below a critical temperature, Tc. A variety of copper oxide materials have been observed to exhibit superconductivity at relatively high temperatures, i.e., above 77K. Since the discovery of the copper oxide-based superconductors, their physical and chemical properties have been widely studied and described in many publications, too numerous to be listed individually.
Composites of superconducting materials and metals are often used to obtain better mechanical and electrical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by a well-known process which includes the stages of: (a) forming a powder of superconductor precursor material (precursor powder formation stage); (b) filling a metal container, such as a tube, billet or grooved sheet, with the precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more cores (filaments) of superconductor precursor material in a surrounding metal matrix (composite precursor fabrication stage); and (c) further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing).
In order to be useful for the majority of applications, substantially single phase superconducting materials with high critical current densities (Jc) are needed. The current-carrying capacity of a superconducting oxide depends significantly upon the degree of alignment and connection of the superconducting oxide grains, together known as xe2x80x9ctexturingxe2x80x9d. The processing of such high-performance superconducting materials may be constrained by the necessity of texturing and densifying the material in order to achieve adequate critical current density. The need to add additional processing steps into the manufacture of an oxide superconductor article in order to achieve adequate critical current density adds significantly to the cost of the final article.
Known processing methods for texturing superconducting oxide composites include various forms of heat treatments and deformation processing (thermomechanical processing). Certain superconducting oxide grains can be oriented along the direction of an applied strain, a phenomenon known as deformation-induced texturing (DIT). Deformation techniques, such as pressing and rolling, have been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. Heat treatment under conditions which at least partially melt and regrow desired superconducting phases also may promote texturing by enhancing the anisotropic growth of the superconducting grains, a phenomenon known as reaction-induced texturing (RIT).
Typically, density and degree of texture are developed in the article by repeated steps of deformation (to impart deformation-induced texturing) and sintering (to impart reaction-induced texturing). The steps of deforming and sintering may be carried out several times, resulting in a process that is both time consuming and expensive. The process may be designated by the term xe2x80x9cnDSxe2x80x9d, in which xe2x80x9cDxe2x80x9d refers to the deformation step, xe2x80x9cSxe2x80x9d refers to the sintering or heating step and xe2x80x9cnxe2x80x9d refers to the number of times the repetitive process of deformation and sintering are carried out. Typical prior art processes are 2DS or 3DS processes.
It is desirable, therefore, to provide a method for preparing a superconducting article having critical current densities acceptable to the art in fewer steps. It is desirable to minimize the number of process steps required while obtaining an acceptable degree of texture and oxide density. In particular, it is desirable to prepare such a superconducting article using a simplified deformation-sintering process which reduces the amount of processing steps required to obtain a superconducting oxide article having adequate critical current. In determining adequate critical current, price to performance ratio ($/KA.m) should be minimized.
The prior art has investigated many texturing processes, but have been unable to reduce the deformation-sintering process to a single iteration, that is a 1DS process while retaining acceptable electrical properties. Chen et al. in U.S. Pat. No. 5,208,215 reports a method of gradually reducing the thickness of a superconducting oxide tape through at least two pressing steps, each pressing step followed by a heating step at 843xc2x0 C., thus reporting at least a 2DS process. Sumitomo Electric Industries, in New Zealand Application No. 230404, reports on 2DS processes for improving high critical current density, in which a variety of processing conditions such as sintering temperatures and times, cooling rates, percent deformation and deformation loads were investigated.
Hikata et al. in U.S. Pat. No. 5,246,917 reports the use of a 2DS or 3DS process for improving critical current density of an oxide superconducting wire which utilizes second rollers for the second rolling process which are at least 5 cm larger than those used in the first rolling step. Sumitomo Electric Industries in EP 0 504 908 A1 reports a 2DS process in which the rolling operation is carried out with rollers having an increased frictional force.
Likewise, Sumitomo Electric Industries, in EP 0 435 286 A1 (xe2x80x9cEP ""286xe2x80x9d), reports a two-step rolling operation for a monofilament wire. Large reduction rolling drafts are used to form a flattened tape, however, EP ""286 discloses that a subsequent rolling operation at smaller drafts is performed before the final heat treatment, thereby teaching a 2DS process. Further, the thicknesses of monofilament wires make it difficult to obtain crack-free high performance wires in a single rolling operation.
Some 1DS processes have been reported; however, they do not result in oxide superconducting wire having acceptable critical current densities. Takahashi et al. in U.S. Pat. No. 5,093,314 disclose a perovskite oxide superconductor monofilament wire prepared by subjecting the wire to repeated extrusions, followed by rolling and heat treatment (a 1DS process) However, Takahashi et al. report performing deformation operations on fully formed oxide superconductor powders which are known to respond poorly to RIT and DIT processes. The fully formed oxide superconductor is not well suited to RIT because the product oxide has already been formed. For example, critical current densities reported by Takahashi et al. are no greater than 3800 A/cm2 for a BSCCO sample.
In a similar fashion, Sumitomo Electric Industries, in EP 0 281 477 A2, describes a process for manufacture of an oxide superconductor in which the oxide superconductor powder is first formed and then introduced into a metallic tube and plastically deformed under compressive strain. Such a method suffers from Limitations similar to those mentioned above for Takahashi et al.
Common deformation techniques in nDS processes include extrusion, drawing, rolling or pressing. Common measures of the effectiveness of the deformation process are expressed as degree of texture, core hardness and core density. Increased core hardness has been associated with improved texturing and core density. Core hardness is a measurement of the hardness of the material as determined by a standard test, such as an indent test. Core density is the density of the oxide powder. Degree of texturing is represented by a fraction between one and zero, with one representing 100% alignment of the c-axes of the oxide grains, such that their slip planes are parallel.
Uniaxial pressing may be an effective method of both aligning the anisotropic oxide grains and densifying the oxide core. See, Li et al. Physica C 217, 360-366 (1993) and Korzekawa et al. Appl. Superconduct. 2(3/4), 261-270 (1994), incorporated by reference herein. The pressing technique has at least one serious drawback in that it can not be carried out conveniently, uniformly and continuously over long lengths of superconducting material.
Rolling, on the other hand, is well-suited for continuous processing of long lengths of superconducting material, particularly wire or tape. However, the rolling operation may sometimes induce cracks and shear bands of the oxide filaments in a direction disruptive to current flow. Further, certain rolling conditions lead to the undesirable distortion of the oxide/metal interface, known as xe2x80x9csausagingxe2x80x9d. See, Li et al. This type of deformation is illustrated FIG. 1, in which dark regions 10 represent oxide filaments and lighter regions 12 represent a surrounding metal matrix. Under certain rolling conditions, an interface 14 of the composite is distorted into a rolling, wavy conformation, resulting in alternating narrow regions 18 and wide regions 16 in the oxide filaments 10.
Cracking formation of shear bands and sausaging may be reduced in a rolling process by the use of extremely low reduction drafts. Typically, multiple reductions on the order of 3% to 5% per pass are used. See, Korzekwa et al., Id. While such low reduction passes may reduce sausaging, it has not been shown to eliminate it entirely. Additionally, low reduction passes exert only a small working force on the composite so that core density, texture and hardness remain very low. For example, the degree of texturing is only around 0.6. Also important, low reduction drafts require multiple processing steps to obtain suitably thin dimensions, thereby increasing the chances of material mishandling and driving up processing costs.
EP 0 435 286 by Sumitomo Electric Industries (EP ""286) describes the preparation of monofilament superconducting oxide wire by flat working the wire at a draft of 80% to 98% reduction in thickness with rollers of 150 mm (7xe2x80x3) diameter, followed by subsequent heat treatment and low reduction rolling. EP ""286 reports an increase in critical current density and suggests that it may be due to improvements in density and orientation, but no mention is made of cracks or rolling defects, such as sausaging. Investigation by applicants has established that roll working of monofilaments is less sensitive to sausaging. Therefore, EP ""286 does not address the sausaging problem in multifilamentary systems.
It has been observed that preferred alignment of superconducting oxide grains can be obtained by the growth of oxide grains in long, thin filaments constrained within a metal matrix. The application of constrained growth of oxide grains is set forth in greater detail in U.S. Ser. No. 08/059,871 filed May 19, 1993, which is a file wrapper continuation of U.S. Ser. No. 07/686,792, now abandoned, entitled xe2x80x9cA Method of Producing Textured Superconducting Oxide Bodies by the Oxidation/Annealing of Thin Metallic Precursorsxe2x80x9d, which are incorporated by reference in its entirety herein. The application describes superconducting oxide compositions having improved crystallographic alignment by constraining the oxide filament diameter to a dimension on the order of the longest dimension of the oxide superconductor grain. It is therefore desirable to prepare superconducting articles having such constrained dimensions so as to take advantage of constrained grain growth.
As discussed extensively above, however, deformation rolling introduces structural defects or sausaging which destroy the constrained volume, at least locally (see, FIG. 1). When rolling operations are carried out on compositions containing such long, thin filaments, xe2x80x9csausagingxe2x80x9d occurs which results in localized filament diameters which are considerably greater than desired. In those regions, texturing and core density is reduced, as described above. FIG. 2 schematically illustrates the effect that the narrowing and expansion of the filament thickness along the filament length has on the alignment of grains of a constrained multifilamentary oxide superconducting article 40. Article 40 includes regions 42 having a dimension sufficient to constrain grain growth to substantially parallel to the constraining dimension, so as to provide dense, aligned oxide grains 44. Article 40 also contains regions 46 which are of significantly greater dimension. Region 46 is typically introduced into the article 40 in rolling deformation processes. The dimension of region 46 is too large to constrain grain growth to a particular orientation. As a result, oxide grains 48 are randomly oriented. Porosity of region 46 increases significantly because of the reduced packing efficiency of the randomly oriented grains.
Therefore, there remains a need to develop deformation processes that avoid cracking, shear band formation and sausaging, while imparting the desired degree of texturing, core density and hardness to a multifilamentary superconducting oxide article. Multifilamentary articles are more useful than monofilamentary articles for mechanical properties and AC applications. This invention enables high-performance multifilamentary oxide superconductor articles having superior performance as compared to prior art mono- and multifilament systems.
It is an object of the present invention to reduce the number of iterative deformation-sintering steps in the processing of an oxide superconductor article.
It is an object of the present invention to provide a method for forming a superconducting article having fine and uniform superconducting oxide filaments without cracks, band shear or sausaging.
It is a further object of the present invention to provide a method for preparing a superconducting oxide article having uniformly high degree of texturing and density using a high reduction deformation process.
It is a further method of the present invention to provide a method for deforming a multifilament superconducting article which optimizes the degree of texturing and core density in a minimum of processing steps.
It is an object of the present invention to provide a method for preparing a superconducting article having high critical current densities using fewer processing steps, namely a high reduction deformation step immediately preceding a single sintering process.
In one aspect of the present invention, a method for preparing a multifilamentary superconducting oxide article is provided. A precursor article is provided which includes a plurality of filaments including a precursor oxide in stoichiometric proportions to form an oxide superconductor, the filaments extending along the length of the article, and a constraining member substantially surrounding the filaments. The precursor article is roll worked in a high reduction draft in the range of about 40% to 95% in thickness to reduce the thickness of the constrained filament to a dimension of less than or substantially equivalent to a longest dimension of the oxide superconductor grains, and the rolled article is sintered to obtain the oxide superconductor.
In another aspect of the invention, a precursor article is provided which includes a plurality of filaments, the filaments including an precursor oxide in stoichiometric proportions to form an oxide superconductor. The filaments extend along the length of the article, and a constraining member substantially surrounds the filaments. The precursor article is roll worked in a high reduction draft in the range of about 40% to 95% in thickness, no further reduction in excess of about 5% occurring after the high reduction roll working step and prior to a sintering step, and the rolled article is sintered to obtain the oxide superconductor.
In yet another aspect of the present invention, a precursor article is provided which includes a plurality of filaments, the filaments including a precursor oxide in stoichiometric proportions to form an oxide superconductor. The filaments extend along the length of the article, and a constraining member substantially surrounds the filaments. The precursor article is roil worked in a high reduction draft in the range of about 40% to 95% in thickness, and the rolled article is sintered to obtain the oxide superconductor.
In one aspect of the invention the constrained filament has a dimension substantially equivalent to a longest dimension of the oxide superconductor grains after roll working. The constraining dimension is less than 100 xcexcm and preferably less than 10 xcexcm. The constraining dimension may include filament thickness or filament width. In another aspect of the invention, no further reduction of the article in excess of about 5% occurs after the high reduction roll working step and prior to a sintering step.
By xe2x80x9cdraftxe2x80x9d as that term is used herein, it is meant the reduction in thickness of an elongated superconducting article in a single deformation operation.
By xe2x80x9croll workingxe2x80x9d, as that term is used herein, it is meant the process of passing a round wire or rectangular tape through the constrained gap of one or more, i.e., a pair of, rollers, so that deformation results.
By xe2x80x9cno further reduction of the article in excess of about 5% occurs after the high reduction roll working stepxe2x80x9d, it is meant that no other deformation processing occurs after the high reduction rolling and before the sintering step. However, other processing operations may be contemplated at this stage of the 1DS process including an ODS (oxide dispersion strengthening) treatment, cabling, coiling winding or other mechanical processing, or conversion of a portion of the precursor oxide into the high temperature superconducting oxide of choice. A xe2x80x9cportionxe2x80x9d will generally include less than 25% of oxide volume.
In preferred embodiments, the high reduction draft is in the range of 40% to 95% in thickness and preferably in the range of 75% to 90% in thickness and preferably in the range of 60% to 90%. In another preferred embodiment, roll working includes rolling the article in a plurality of drafts to reduce thickness and subjecting the article to a final draft before heat treatment, the final draft including a high reduction draft in the range of about 40% to 95% in thickness. The plurality of drafts prior to final draft may include drafts in the range of 1% to 10% or additional high reduction drafts.
In other preferred embodiments, the constraining member includes a noble metal. Noble metal, as that term is used herein, is ment to include any metal or an alloy including a noble metal which is unreactive to the oxide superconductor or its precursors, under the reaction described herein, in particular the noble metal is substantially inert to oxidation under conditions of high temperature superconductor formation. Typical noble metals include, silver, gold, platinum and palladium, and alloys thereof.
In another aspect of the invention, the manufacture of a BSCCO 2223 superconducting oxide article includes subjecting an oxide article comprised of predominantly a BSCCO 2212 phase to a heat treatment at an oxygen partial pressure and temperature capable of modifying the phase composition of the oxide article. The heat treatment occurring prior to deformation and sintering of the oxide article. xe2x80x9cPredominantlyxe2x80x9d is used herein to mean at least 50% and preferably greater than 90% of the named phase.
In another aspect of the invention, the manufacture of a BSCCO 2223 superconducting oxide article includes subjecting an oxide article comprised of a mainly a BSCCO 2212 phase to a heat treatment at an oxygen partial pressure in the range of about 10xe2x88x925 to 10xe2x88x922 atm O2 and at a temperature in the range of about 600xc2x0 C. to 850xc2x0 C. The heat treatment occurring prior to deformation and sintering of the article.
In yet another aspect of the invention, the manufacture of a BSCCO 2223 superconducting oxide article includes subjecting a precursor to an oxide superconducting article to a heat treatment at an oxygen partial pressure in the range of about 10xe2x88x925 to 10xe2x88x922 atm O2 and at a temperature in the range of about 600xc2x0 C. to 850xc2x0 C., said heat treatment occurring prior to deformation and sintering of the article.
In preferred embodiments, the oxygen partial pressure and temperature are selected such that the phase modification comprises conversion of a tetragonal phase BSCCO 2212 into an orthorhombic phase BSCCO 2212. The oxygen partial pressure may be in the range of about 10xe2x88x925 to 10xe2x88x922 atm O2 and the temperature is in the range of about 600xc2x0 C. to 850xc2x0 C., and preferably in the range of about 700xc2x0 C. to 800xc2x0 C. and preferably in the range of about 750-790xc2x0 C.
In other preferred embodiments, prior to heat treatment of the oxide article, a multifilamentary oxide article is provided which includes a plurality of oxide filaments comprised of mainly a BSCCO 2212 phase, each of the filaments extending along the length of the article, and a constraining member substantially surrounding each of the filaments, wherein a constraining dimension is substantially equivalent to a longest dimension of the oxide superconductor grains. The oxide article prior to phase modification may comprise a dominant amount of a tetragonal BSCCO 2212 oxide.
In other preferred embodiments, the method further includes roll working the heat treated oxide article, and preferably includes roll working the phase modified oxide article in a high reduction draft in the range of about 40% to 95% in thickness, and preferably includes rolling the heat treated oxide article in a plurality of drafts, and subjecting the rolled phase modified oxide article to a high reduction draft in the range of about 40% to 95% in thickness.
In another aspect of the invention, an oxide superconductor article is prepared in a 1DS process which includes a precursor article having a plurality of filaments containing a precursor oxide in stoichiometric proportions to form an oxide superconductor, the filaments extending along the length of the article, and a constraining non-superconductive boundary member substantially surrounding the filaments, wherein a constraining dimension is substantially equivalent to a longest dimension of the oxide superconductor grains. The precursor oxide article is subjected to a heat treatment at an oxygen partial pressure and temperature capable of modifying the phase composition of the oxide precursor and roll worked in a high reduction draft in the range of about 40% to 95% thickness, reduction draft occurring just prior to sintering. The rolled article is sintered to obtained the oxide superconductor. In preferred embodiment, the precursor oxide is comprised of mainly a tetragonal BSCCO 2212 oxide.
By xe2x80x9csingle sintering processxe2x80x9d it is meant, processing at a temperature or a series of temperatures, without further deformation texturing and sintering processing.
By xe2x80x9csinteringxe2x80x9d or xe2x80x9csintering heat treatmentxe2x80x9d as that term is used herein, it is meant a heat treatment by which oxides are converted into the high temperature oxide superconductor of choice and by which oxide grains are bonded together.
In another aspect of the invention, a BSCCO 2223 oxide superconductor article is prepared in a 1DS process by a filing a plurality of metal tubes with a predominant amount of orthorhombic BSCCO 2212 oxide; and reducing the diameter of each orthorhombic BSCCO 2212-filled metal tube and redrawing bundles of each of the orthorhombic BSCCO 2212-filled metal tube, so as to form a multifilamentary orthorhombic BSCCO 2212 oxide article constrained by the metal, wherein the plurality of filaments extend along the length of the article and wherein the constraining metal substantially surrounds each of the filaments, such that a constraining dimension is substantially equivalent to a longest dimension of the oxide superconductor grains. The orthorhombic BSCCO 2212 oxide article is roll worked in a high reduction draft in a range of about 40% to 95% in thickness, the high reduction draft occurring just prior to sintering. Thereafter, the rolled article is sintered to obtain the BSCCO 2223 oxide superconductor.
In another aspect of the present invention, a BSCCO 2223 oxide superconductor article is prepared by a 1DS process which includes providing a precursor article including a plurality of filaments including a precursor oxide comprised of mainly a tetragonal BSCCO 2112 phase, each of the filaments extending along the length of the article, and a constraining member substantially surrounding each of the filaments. The precursor oxide article is subjected to a heat treatment at an oxygen partial pressure and temperature selected to convert a tetragonal BSCCO 2212 phase into an orthorhombic BSCCO 2212 phase. The orthorhombic BSCCO 2212 oxide-containing article is roll worked in a high reduction draft in a range of about 40% to 95% in thickness, and the rolled article is sintered to obtained a BSCCO 2223 oxide superconductor.
In a preferred embodiment, the step of roil working is carried out to reduce the thickness of the constrained filament to a dimension of less than or substantially equivalent to a longest dimension of the oxide superconductor grains. The step of roil working alternatively is carried out such that no further reduction in excess of about 5% occurs after high reduction roll working and prior to sintering. In another preferred embodiment, the precursor oxide is comprised of a predominant amount of tetragonal phase BSCCO 2212. In other preferred embodiments, the high reduction draft is in the range of 75% to 90% in thickness, and preferably roll working includes rolling the article in a plurality of drafts, and subjecting the article to a high reduction draft in the range of about 40% to 95% in thickness.
In another aspect of the invention, an oxide superconductor article is prepared in a 1DS process by providing a precursor article which includes a plurality of filaments comprised of a precursor oxide in stoichiometric proportions to form an oxide superconductor, the filaments extending along the length of the article, and a constraining member substantially surrounding the filaments. The precursor oxide article is subjected to a heat treatment at an oxygen partial pressure in the range of about 10xe2x88x925 to 10xe2x88x922 atm O2 and at a temperature in the range of about 600xc2x0 C. to 850xc2x0 C. The heated precursor article is roll worked in a high reduction draft in the range of about 40% to 95 % in thickness, and then sintered to obtain the oxide superconductor.
In preferred embodiments, sintering includes heating at a first temperature in the range of 810xc2x0 C. to 850xc2x0 C., heating at a second temperature in the range of 800xc2x0 C. to 840xc2x0 C., and heating at a third temperature in the range of 700xc2x0 C. to 800xc2x0 C. Sintering includes heating at an oxygen partial pressure of about 0.0001 to 0.5 atm.
In yet another aspect of the present invention, a multifilamentary oxide superconductor article is provided which includes a plurality of filaments containing an oxide superconductor. The filaments extend along the length of the article. The article also includes a constraining member substantially surrounding each of the filaments. Each of the filaments has an average transverse cross-section less than about 35 xcexcm and an average variation in cross-section along its length of less than about 15%. In preferred embodiments, each of the filaments has an average transverse cross-section less than about 20 xcexcm and an average variation in cross section along its length of less than about 10%. The oxide superconductor is comprised of BSCCO 2223 in a preferred embodiment, but may also be comprised of any of the known superconductor oxide families.
In another aspect of the invention, multifilamentary oxide article is provided which includes a plurality of filaments containing an oxide with the filaments extending along the length of the article. The article also includes a constraining member substantially surrounding each of the filaments. Each of the filaments has an average transverse cross-section less than about 35 xcexcm and an average variation in cross section along its length of less than about 10%. In other preferred embodiments, each of the filaments has an average transverse cross-section less than about 20 xcexcm and an average variation in cross section along its length of less than about 5%. The oxide is comprised of a BSCCO 2212 phase in a preferred embodiment, but may also be comprised of any oxide.
In other preferred embodiments, the article includes about 10 to 10,000 filaments per article, and preferably 10 to 1000 filaments per article. The oxide filaments of the article may have a hardness of greater than 200 KHN. In other preferred embodiments, the article has a constraining dimension of less than 10 xcexcm.
The multifilamentary article includes a plurality of filaments comprised of perdominantly BSCCO 2212 phase, each of the filaments extending along the length of the article, and a constraining member substantially surrounding each of the filaments, wherein a constraining dimension is substantially equivalent to a longest dimension of the oxide superconductor grains. In other preferred embodiments, article is a multifilamentary tape having filaments in the range of 10 to 10,000 filaments per article, and preferably 10 to 1000 filaments pre article.
In yet another aspect of the invention, a multifilamentary oxide article is provided including a plurality of filaments including an oxide, the filaments extending along the length of the article, and a constraining member substantially surrounding each said filament, said constraining member having a constraining dimension of less than or equal to about 10 microns. Each of said filament is comprised of oxide grains of less than about 5 microns in at least one dimension.
The method of the present invention provides an oxide article having superior electrical properties as compared to those conventionally prepared articles. Additionally, the method of the present invention represents a great improvement in the number and cost of processing steps necessary to obtain a high quality oxide superconductor. The article of the present invention may be used in a variety of applications, including, but not limited to the transmission of electricity and operation of electric motors.